(1) Field of the Invention
This invention relates to braking hydraulic pressure control units for motor vehicles, and more particularly to improvements in or relating to a braking hydraulic pressure control unit of the deceleration sensing type mounted in a hydraulic pressure circuit connecting a master cylinder to rear wheel cylinders and responsive to the degree of deceleration of the vehicle for controlling a braking hydraulic pressure introduced into the rear wheel cylinders when the brake is applied.
(2) Description of the Prior Art
It is theoretically known that in order to evenly decelerate front wheels and rear wheels of a motor vehicle to stably control brake application when the brake is applied, it is necessary to control a braking hydraulic pressure in such a manner that an increase in the braking hydraulic pressure supplied from a master cylinder to rear wheel cylinders of a vehicle braking system is gradually reduced in comparison with a rise in the braking hydraulic pressure supplied to front wheel cylinders. As one type of braking device which enables the aforesaid control to be effected, a braking system is widely known which includes a braking hydraulic pressure control valve of the deceleration sensing type mounted in a circuit connecting the master cylinder to the rear wheel cylinders. The braking hydraulic pressure control valve of the type described which is widely used nowadays has a spherical valve floatingly contained in a valve chamber communicating with the master cylinder and the rear wheel cylinders, and a differential-acting piston which bears the hydraulic pressure on its master cylinder side and the hydraulic pressure on its rear wheel cylinder side. This valve is characterized in that when the deceleration of a motor vehicle reaches a predetermined level at the time of brake application, the spherical valve is moved by inertia in a vehicle traveling direction to close a spherical valve port to thereby interrupt communication between the master cylinder and the rear wheel cylinders and thereafter to raise the pressure of the braking fluid on the rear wheel cylinder side, which is thus separated from the braking fluid on the master cylinder side, at a predetermined reduced rate by the action of the hydraulic pressure on the master cylinder side through the agency of the differential-acting piston. Thus, until the spherical valve port is closed the pressure of the braking fluid in the rear wheel cylinders raises at the same rate as the pressure of the braking fluid in front wheel cylinders which are in communication with the master cylinder, and after the spherical valve port is closed the pressure of the braking fluid in the rear wheel cylinders rises at a lower rate than the pressure of the braking fluid in the front wheel cylinders. By this arrangement, the distribution of a braking force between the front wheels and the rear wheels of the vehicle can be expressed by a line which is bent at a dividing point corresponding to the point in time of the closure of the spherical valve port, and approximated to a theoretically ideal braking force distribution.
The hydraulic pressure at this dividing point varies depending on the size of shoe clearances of rear brakes. Generally, the dividing point tends to become low when the shoe clearances increase in size and to become high when they decrease in size. This phenomenon is caused by the fact that a dynamic pressure produced by the flow of the braking fluid is applied to the spherical valve.